1. Technical field
The present invention generally relates to a coupling or connecting arrangement and more particularly, to a bayonet mount arrangement as a coupling between mating optical units in an optical instrument such as a lens interchangeable photographic camera and the like.
2. Prior art
In FIGS. 1 through 6, there are shown conventional examples of photographic cameras which employ bayonet mount arrangements of this kind.
FIG. 1 shows the relationship before meshing immediately after insertion of bayonet claws at one side into mating bayonet recesses, between a mount base 1 of a camera main body and a mount portion 2 of an interchangeable lens unit (referred to merely as a lens unit hereinafter). In the drawing, numerals 1a designates the bayonet claws of the camera main body, 1b the bayonet recesses of the camera main body, 2a the bayonet claws of the lens unit, and 2b the bayonet recesses of the lens unit. The bayonet claws 1a of the camera main body are disposed separately at three places along the inner peripheral edge of the base mount 1 of the camera main body, while, at the remaining three places along the inner peripheral edge of the mount base 1 between the respective bayonet claws 1a, the bayonet recesses 1b are provided. Similarly, in the lens unit also, the bayonet claws 2a are formed at three places along the outer peripheral edge of the mount portion 2 in positions respectively corresponding to the bayonet recesses 1b of the camera main body, with bayonet recesses 2b being disposed at the remaining three placed along the peripheral edge of the mount portion 2 in positions respectively corresponding to the bayonet claws 1a of the camera main body. In dimensions, the width of each bayonet claw 2a of the lens unit is slightly smaller than that of the corresponding bayonet recess 1b, while the width of each bayonet recess 2b of the lens unit in the circular direction is set to be slightly larger than that of the corresponding bayonet claw 1a of the camera body, whereby when the mount portion 2 of the lens unit is inserted into the mount base 1 of the camera main body in a regular corresponding relation as shown in FIG. 1, the bayonet claws 1a and 2a of the both sides do not interfere with each other, whereas when the mount portion 2 of the lens unit is intended to be inserted into the mount base 1 of the camera main body 1 at a different angular position, i.e. in an irregular corresponding relation, the bayonet claws 1a and 2a interfere with each other so as to prevent the insertion.
On the other hand, in a connection or coupling completed state as shown in FIG. 2 in which the mount portion 2 of the lens unit is mounted on the mount base 1 of the camera main body, there is provided a small clearance .DELTA. between a rear face 1d of the bayonet claw 1a of the camera main body and the confronting rear face 2d of the bayonet claw 2a of the lens unit. The clearance is provided to permit the rotational mounting operation of the lens unit onto the camera main body while achieving the bayonet engagement. Moreover, between the mount base 1 of the camera main body and a front frame 3 of the camera main body on which mount base 1 is secured, a pressure spring 4 is disposed, with a spring portion thereof being disposed at a notched portion 1e formed at part of the rear face 1d of the bayonet claw 1a of the camera main body. Under the influence of pressure spring 4 as described above, in the course of rotation of the lens unit for the mounting thereof onto the camera main body, and also, in the engagement completed state, the bayonet claws 2a of the lens unit are depressed towards the camera main body side, so that any looseness or play resulting from the small clearance .DELTA. between the corresponding bayonet claws 1a and 2a are advantageously avoided, thus making it possible to maintain corresponding seat faces 1c and 2c in a state tightly contacting each other. Furthermore, the pressure spring 4 is formed with projections (not shown) for restricting an attaching and detaching rotational angle, adapted to protrude towards the bayonet claws 2a of the lens unit.
In the known bayonet mount arrangement as described so far, when the mount portion 2 of the lens unit is rotated up to a mount lock position (not shown) in an engaging direction (i.e. clockwise in FIG. 1), from the state as shown in FIG. 1 in which the mount portion 2 of the lens unit is inserted into the mount opening of the camera main body, with a mounting pointer mark (not shown) marked in the vicinity of the mount portion 2 of the lens unit being aligned with a corresponding mounting pointer mark (not shown) located close to the mount base 1 of the camera main body, the corresponding bayonet claws 1a and 2a are engaged with each other between the mount base 1 and the mount portion 2 at the three places as shown by hatching in FIG. 2, and thus, the connection between the camera main body and the lens unit is completed. In the above connection completed state, the seat faces 1c and 2c are held in close contact with each other not only at the portions where the bayonet claws 1a and 2a of the camera main body and the lens unit are in mesh with each other as shown in FIG. 3, but also, at the portions where the bayonet claws 1a and 2a are not present as shown in FIG. 4.
However, in the prior art bayonet mount arrangement as described in the foregoing, of the engaging portions between the bayonet claws 1a and 2a in the above mounting completed state, the two engaging portions, except for the engaging portion located at the upper peripheral part of the mount base 1 and the mount portion 2 right above the center 0, are positioned at the lower side peripheral parts of the mount base 1 and the mount portion 2 lower than the lateral side peripheral parts thereof diametrically opposite to one another with respect to the center 0 in the lateral direction when observed from the front side, with the camera being placed at a normally erect attitude (i.e. a posture for ordinary use), and this results in disadvantages as described hereinbelow. Meanwhile, the center 0 is the center of the mount base 1 which aligns with the optical axis of the camera and also the center of the mount portion 2 which aligns with the optical axis of the lens unit.
By way of example, when a heavy and long interchangeable lens barrel such as a telephoto lens L or the like is mounted on the camera body, the load or weight thereof is applied to the bayonet mount portion, and thus, due to the presence of the earlier described small clearance .DELTA. provided between the rear face 1d of the bayonet claw 1a of the camera main body and the rear face 2d of the lens unit bayonet claw 2a, lens unit L is tilted downwardly as shown in FIG. 5 in an exaggerated manner. Therefore, the seat faces 1c and 2c are held in contact with each other at the lower side peripheral parts of the mount base 1 and the mount portion 2, while, at the upper peripheral part, the bayonet claws 1a and 2a are brought into direct contact with each other against the pressure spring 4, and consequently, the seat faces 1c and 2c are held in a state spaced from each other.
Accordingly, in photographing in which the bayonet mount portion is held in the normally erect attitude as shown in FIG. 2, i.e. in the shooting with the camera held horizontally, the bayonet claws 1a and 2a contact each other at the upper peripheral part of the mount base 1 and the mount portion 2.
In the above case, on the assumption that the spacing amount between the seat faces 1c and 2c at the uppermost position of the upper peripheral part is represented by .delta., the distance from the lowermost position of the lower side peripheral parts where the seat faces 1c and 2c contact each other to the optical axis of the camera main body is denoted by R, and the distance from the uppermost position of the upper peripheral part where the bayonet claws 1a and 2a contact each other to the optical axis is shown by r, the angle .theta..sub.1 of inclination of the lens unit with respect to the camera at such horizontally held position may be represented by tan.sup.-1 .delta./r+R.
In contrast to the above, in shooting with the camera held vertically, wherein the cross section taken along the line IV--IV in FIG. 2 is directed vertically, the upper most position of the upper peripheral part of the mount base 1 and the mount portion 2 where the bayonet claws 1a and 2a overlap each other at this time, is located at a position spaced an angle by .alpha. from the line IV--IV. The spacing amount between the seat faces 1c and 2c at this position is also .delta.. This position will be represented by r.multidot.cos .alpha. when converted into a distance l in the vertical direction from the axis in case of the shooting with the camera held vertically. Accordingly, the angle .theta..sub.2 of the inclination of the lens barrel with respect to the camera main body at this shooting with the camera held vertically, may be denoted by tan.sup.-1 .delta./r cos .alpha.+R.
This means that the inclination of the lens barrel with respect to the camera main body is increased as the angle .alpha. increases, i.e. as the engaging position of the bayonet claws 1a and 2a is spaced from the position immediately above the center 0, and also that the inclination of the lens optical axis is reduced as the diameter of the bayonet mount portion becomes larger.
As is seen from the foregoing description, in the conventional bayonet mount arrangement as explained so far, the inclination of the lens system of the lens unit with respect to the film surface inside the camera main body is increased in photographing with the camera held vertically, thus resulting in the so-called "one sided out-of-focus" so as to deteriorate the photographing performance (i.e., image definition). Therefore, as a countermeasure against such an inconvenience, it becomes necessary to increase the degree of close contact between the seat faces 1c and 2c by increasing the force urging rear face 2d of the bayonet claw 2a of the lens unit towards the camera main body side, with the urging force of the present spring 4 being increased. On the other hand, however, if the degree of close contact between the seat faces 1c and 2c is increased, the respective seat faces 1c and 2c tend to be easily injured by the sliding movements thereof during the rotation in the attaching or detaching of the lens unit onto or from the camera main body. There are also invited such problems that when the pressure on the rear face 2d of the bayonet claw 2a of the lens unit is increased, the contact face of the pressure spring 4 scrapes the rear face 2d of the bayonet claw 2a, giving rise to the formation of abraded particles, and furthermore that the torque required during the mounting or demounting is undesirably increased, with a deterioration in the operability during exchanging of interchangeable lens units.